In a variety of industrial fields, for example those involved in the manufacturing of devices for the administration of medicinal compounds, it is desirable to find means for nebulising a fluid or liquid in a controlled manner.
One known solution to this problem involves the agitation of a membrane by means of a piezoelectric oscillator, wherein the fluid to be nebulised is placed at one side of said membrane such that the fluid is nebulised in a controlled manner to provide fine liquid droplet sprays, mists or aerosols on the other side of the membrane.
Such a device, as for example known from EP 0 615 470 A, commonly comprises an annular substrate, on one side of which is disposed an annular piezoelectric device, and on the other side of which is placed a circular dispersion element, all three elements being disposed coaxially. The circular dispersion element may comprise a plurality of fine diameter holes, substantially parallel to the axis of the device, through which fluid passes to form droplets. A device of this kind is shown in FIG. 1A, while FIG. 1B shows a cross-section of the device of 1A along the lines of AA and how on the energizing of the piezoelectric element 4, a standing wave is set up through the device as a whole, having maxima at the centre of the device, where the dispersion element or mesh 10 is located, and side lobes nearer the circumference of the substrate.
A problem arises with the described device where it is necessary to fixedly attach it to a housing. The ways of doing this known in the prior art either result in an undesirable influence on the oscillation characteristics of the device or are difficult to handle, especially in the case of a medical treatment device employing such a device.
One known method of securing the nebuliser known in the prior art, as indicated in FIG. 1B, involves the provision of bearings 20, 21, 22 and 23 positioned so as to support the substrate 3 at points at which the standing wave present under oscillating conditions is known to be a minimum. Clearly, this solution is limited by practical considerations, such that in practice a significant deterioration in the performance of the nebuliser is experienced.
An alternative method known in the prior art, as indicated in FIG. 1C, involves the clamping of the substrate 3 at an circumferential edge, involving using fixed clamps 24. Once again, this results in a significant deterioration in the performance in the nebuliser. If the clamping is achieved by means of a ring of a soft resilient material the small gaps between the ring 24 an the substrate 3 are prone to collect fluids or other substances causing hygiene problems.
As mentioned above, these and other prior art methods of fixedly securing the nebuliser device are disclosed in EP 0 615 470 A.
A further problem arising in these known techniques is that there are provided numerous devices which are prone to retention of stray matter in an undesirable manner, particularly in medical applications where hygiene is of particular importance.